In continuous casting equipment, molten metal is poured into a mold, cooled and solidified there, such that a desired shape of strand is produced. In continuous casting equipment, measurement and control of a level of a surface of molten metal in the mold, that is, a mold level are essential for improving quality of a produced strand.
Eddy current mold level measuring devices measure a level of a surface of molten metal in the mold by the use of the phenomenon that magnitude of voltage which is induced in a detecting coil by eddy current generated around a surface of molten metal in the mold varies depending on a distance between the detecting coil and the surface of molten metal. Eddy current mold level measuring devices are excellent in responsivity and therefore appropriate for a high-accuracy control of a level of a surface of molten metal in the mold, but are susceptible to ambient temperature and electromagnetic field which surrounds the devices. Accordingly, calibration is essential to eddy current mold level measuring devices. As methods for calibrating conventional eddy current mold level measuring devices, a method which uses measurements obtained by visual observations of the operator (for example, JPS61239120(A)), a method which uses a thermocouple type mold level meter (for example, JPH02140621(A)), a method which uses an electrode type mold level meter (for example, JPH08233632(A)), and the like have been developed. However, any of the above-described methods are insufficient in accuracy and are not adaptable to dynamic disturbances such as a change in vertical position of a tundish and a change in width of a slab mold. In particular, in the case of small-section molds, such as those for bloom and billet, eddy current mold level measuring devices are significantly affected by a change in vertical position of the tundish, and the change in vertical position of the tundish has been an obstacle to improvement of accuracy of the measuring devices.
Further, a method in which characteristics of a mold level measuring device are determined by the use of signals generated by mold oscillation has been proposed (JPS60216959(A)). However, the method disclosed in JPS60216959(A) has problems as described below. Since mold level measuring devices are used for control of pouring rate of molten metal into the mold, an error in measurement may lead to a serious accident. Accordingly, when calibration of a mold level measuring device is carried out for the duration of continuous casting process, safety and reliability of the calibration has to be ensured. However, JPS60216959(A) does not say anything about how calibration of a mold level measuring device should be carried out while safety and reliability of the calibration are ensured for the duration of continuous casting process. Accordingly, the method disclosed in JPS60216959(A) cannot be brought into practical use in calibration for the duration of continuous casting process. In the method disclosed in JPS60216959(A), feedforward-type correction is made by a correction circuit 12 as shown in FIG. 4, and a positive feedback ratio is not corrected unlike the present invention. The present invention will be described in detail later.
Thus, any of the conventional methods cannot provide a sufficient degree of accuracy of measurement while adapting to changes in surrounding conditions in the continuous casting process.
Patent document 1: JPS61239120(A)
Patent document 2: JPH02140621(A)
Patent document 3: JPH08233632(A)
Patent document 4: JPS60216959(A)
Accordingly, there is a need for an eddy current mold level measuring device and a mold level measuring method in which calibration of the mold level measuring device can be carried out while safety and reliability of the calibration are ensured for the duration of continuous casting process and a sufficient accuracy of measurement can be guaranteed.